1. Field of the Invention
The present invention relates to a corona discharge device for uniformly charging a dielectric surface and, more specifically, to an improvement of a corona discharge device preferably used in an electrophotographic apparatus.
2. Description of the Related Art
A corona discharge device may be used as a charging device for generating a prescribed electrostatic potential on an image forming surface of an electrophotographic apparatus, for example. In one example of a conventional corona discharge device, a high voltage of 5 to 10 kV is applied to a large number of tungsten wires having the diameter of 50 to 100 .mu.m, ions generated by discharge from these wires are moved onto the image forming surface, whereby the image forming surface is charged.
However, when negative discharge is carried out in this wire type corona discharge device, discharge takes place at random points on these wires dependent on the states of the number of wires, resulting in uneven and instable discharge with respect to the dielectric surface. Therefore, in order to uniformly charge the dielectric surface, a shield case as an auxiliary electrode or a grid electrode for controlling potential is used. However, despite of such improvements, much discharging current must be used in the wire type corona discharge device in order to obtain good stability and uniformity of charges. As a result, amount of ozone generated in the electrophotographic apparatus increases, causing degradation of image quality and possible adverse effect on human body.
Meanwhile, recently, a corona discharge device has been proposed in which a saw-tooth or needle like discharge electrode is used instead of the tungsten wires, as disclosed, for example, in Japanese Patent Laying-Open No. 63-15272. In the saw-tooth type corona discharge device, discharge points are regularly arranged at tips of a plurality of saw teeth, and therefore discharge becomes more uniform with respect to the dielectric surface. In addition, in the saw-tooth type discharge device, discharge current necessary for generating uniform static electrification is smaller than in the wire type discharge device, structural strength is relatively high, and the amount of undesirable ozone generated can be reduced.
FIG. 20 schematically shows a conventional corona discharge device. In the corona discharge device, a saw-tooth discharge electrode 51 formed of stainless steel is mounted on an insulator substrate 52. Saw-tooth discharge electrode 51 includes 10 electrode teeth 51a arranged at a pitch of 2 mm. Opposing to saw-tooth discharge electrode 51, a counter electrode 53 formed of stainless steel is placed spaced apart by a prescribed distance g from the tips of electrode teeth 51a. A high voltage source 54 is connected to saw-tooth discharge electrode 51. By applying a high voltage from high voltage source 54 to saw-shaped discharge electrode 51, corona discharge occurs from the tips of electrode teeth 51a to counter electrode 53.
Table 1 shows results of measurement of discharge current flowing through respective electrode teeth 51a when discharge takes place in the corona discharge device of FIG. 20. In this measurement, a voltage of -4.3 kV was applied to discharge electrode 51, and the space between discharge electrode 51 and counter electrode 53 was g=7 mm. The left column of Table 1 represents the number of electrode tooth 51a from the left, and the right column represents the discharge current flowing between the corresponding electrode tooth 51a and counter electrode 53.
TABLE 1 ______________________________________ Electrode Tooth No. Discharge Current (from Left) (.mu.A) ______________________________________ 1 1.90.about.2.20 2 0.1 3 0.30.about.0.80 4 1.20.about.1.90 5 1.1 6 0.30.about.0.38 7 0 8 0.48.about.0.54 9 0.18 10 0.80.about.1.20 ______________________________________
In such a corona discharge device as shown in FIG. 20, discharge occurs at equal interval from the tips of electrode teeth 51a arranged at a prescribed pitch. However, as can be seen from Table 1, discharge current from the electrode teeth 51 varies considerably, resulting in instable discharge. Possible cause of such instability of discharge at respective electrode teeth 51a may be variation in fine configuration, defects, contamination and so on at each of the electrode teeth 51a. Accordingly, even when such a saw-tooth discharge electrode as shown in FIG. 20 is used, a considerable discharge current must be used in order to uniformly charge the dielectric surface. Though the amount of ozone generated in the saw-tooth type discharge device can be reduced to one fifth that of the wire type discharge device (when the discharge current is the same between the two types), further reduction of the amount of generated ozone is desired.
Japanese Patent Laying-Open No. 5-2314 teaches a method of improving stability of discharge current in the saw-tooth or needle like type corona discharge device. In this method, each of a plurality of saw tooth or needle like discharge electrodes is connected to a high voltage source through resistor element. However, Japanese Patent Laying-Open No. 5-2314 is silent about what specific resistor element is used, and how such element is formed.